function RenderCache::set

Overrides RenderCacheInterface::set

File

core/lib/Drupal/Core/Render/RenderCache.php, line 88

Class

RenderCache

Wraps the caching logic for the render caching system.

Namespace

Code

public function set(array &$elements, array $pre_bubbling_elements) {
    // Form submissions rely on the form being built during the POST request,
    // and render caching of forms prevents this from happening.
    // @todo remove the isMethodCacheable() check when
    //       https://www.drupal.org/node/2367555 lands.
    if (!$this->requestStack
        ->getCurrentRequest()
        ->isMethodCacheable() || !($cid = $this->createCacheID($elements))) {
        return FALSE;
    }
    $data = $this->getCacheableRenderArray($elements);
    $bin = isset($elements['#cache']['bin']) ? $elements['#cache']['bin'] : 'render';
    $cache = $this->cacheFactory
        ->get($bin);
    // Calculate the pre-bubbling CID.
    $pre_bubbling_cid = $this->createCacheID($pre_bubbling_elements);
    // Two-tier caching: detect different CID post-bubbling, create redirect,
    // update redirect if different set of cache contexts.
    // @see \Drupal\Core\Render\RendererInterface::render()
    // @see ::get()
    if ($pre_bubbling_cid && $pre_bubbling_cid !== $cid) {
        // The cache redirection strategy we're implementing here is pretty
        // simple in concept. Suppose we have the following render structure:
        // - A (pre-bubbling, specifies #cache['keys'] = ['foo'])
        // -- B (specifies #cache['contexts'] = ['b'])
        //
        // At the time that we're evaluating whether A's rendering can be
        // retrieved from cache, we won't know the contexts required by its
        // children (the children might not even be built yet), so cacheGet()
        // will only be able to get what is cached for a $cid of 'foo'. But at
        // the time we're writing to that cache, we do know all the contexts that
        // were specified by all children, so what we need is a way to
        // persist that information between the cache write and the next cache
        // read. So, what we can do is store the following into 'foo':
        // [
        //   '#cache_redirect' => TRUE,
        //   '#cache' => [
        //     ...
        //     'contexts' => ['b'],
        //   ],
        // ]
        //
        // This efficiently lets cacheGet() redirect to a $cid that includes all
        // of the required contexts. The strategy is on-demand: in the case where
        // there aren't any additional contexts required by children that aren't
        // already included in the parent's pre-bubbled #cache information, no
        // cache redirection is needed.
        //
        // When implementing this redirection strategy, special care is needed to
        // resolve potential cache ping-pong problems. For example, consider the
        // following render structure:
        // - A (pre-bubbling, specifies #cache['keys'] = ['foo'])
        // -- B (pre-bubbling, specifies #cache['contexts'] = ['b'])
        // --- C (pre-bubbling, specifies #cache['contexts'] = ['c'])
        // --- D (pre-bubbling, specifies #cache['contexts'] = ['d'])
        //
        // Additionally, suppose that:
        // - C only exists for a 'b' context value of 'b1'
        // - D only exists for a 'b' context value of 'b2'
        // This is an acceptable variation, since B specifies that its contents
        // vary on context 'b'.
        //
        // A naive implementation of cache redirection would result in the
        // following:
        // - When a request is processed where context 'b' = 'b1', what would be
        //   cached for a $pre_bubbling_cid of 'foo' is:
        //   [
        //     '#cache_redirect' => TRUE,
        //     '#cache' => [
        //       ...
        //       'contexts' => ['b', 'c'],
        //     ],
        //   ]
        // - When a request is processed where context 'b' = 'b2', we would
        //   retrieve the above from cache, but when following that redirection,
        //   get a cache miss, since we're processing a 'b' context value that
        //   has not yet been cached. Given the cache miss, we would continue
        //   with rendering the structure, perform the required context bubbling
        //   and then overwrite the above item with:
        //   [
        //     '#cache_redirect' => TRUE,
        //     '#cache' => [
        //       ...
        //       'contexts' => ['b', 'd'],
        //     ],
        //   ]
        // - Now, if a request comes in where context 'b' = 'b1' again, the above
        //   would redirect to a cache key that doesn't exist, since we have not
        //   yet cached an item that includes 'b'='b1' and something for 'd'. So
        //   we would process this request as a cache miss, at the end of which,
        //   we would overwrite the above item back to:
        //   [
        //     '#cache_redirect' => TRUE,
        //     '#cache' => [
        //       ...
        //       'contexts' => ['b', 'c'],
        //     ],
        //   ]
        // - The above would always result in accurate renderings, but would
        //   result in poor performance as we keep processing requests as cache
        //   misses even though the target of the redirection is cached, and
        //   it's only the redirection element itself that is creating the
        //   ping-pong problem.
        //
        // A way to resolve the ping-pong problem is to eventually reach a cache
        // state where the redirection element includes all of the contexts used
        // throughout all requests:
        // [
        //   '#cache_redirect' => TRUE,
        //   '#cache' => [
        //     ...
        //     'contexts' => ['b', 'c', 'd'],
        //   ],
        // ]
        //
        // We can't reach that state right away, since we don't know what the
        // result of future requests will be, but we can incrementally move
        // towards that state by progressively merging the 'contexts' value
        // across requests. That's the strategy employed below and tested in
        // \Drupal\Tests\Core\Render\RendererBubblingTest::testConditionalCacheContextBubblingSelfHealing().
        // Get the cacheability of this element according to the current (stored)
        // redirecting cache item, if any.
        $redirect_cacheability = new CacheableMetadata();
        if ($stored_cache_redirect = $cache->get($pre_bubbling_cid)) {
            $redirect_cacheability = CacheableMetadata::createFromRenderArray($stored_cache_redirect->data);
        }
        // Calculate the union of the cacheability for this request and the
        // current (stored) redirecting cache item. We need:
        // - the union of cache contexts, because that is how we know which cache
        //   item to redirect to;
        // - the union of cache tags, because that is how we know when the cache
        //   redirect cache item itself is invalidated;
        // - the union of max ages, because that is how we know when the cache
        //   redirect cache item itself becomes stale. (Without this, we might end
        //   up toggling between a permanently and a briefly cacheable cache
        //   redirect, because the last update's max-age would always "win".)
        $redirect_cacheability_updated = CacheableMetadata::createFromRenderArray($data)->merge($redirect_cacheability);
        // Stored cache contexts incomplete: this request causes cache contexts to
        // be added to the redirecting cache item.
        if (array_diff($redirect_cacheability_updated->getCacheContexts(), $redirect_cacheability->getCacheContexts())) {
            $redirect_data = [
                '#cache_redirect' => TRUE,
                '#cache' => [
                    // The cache keys of the current element; this remains the same
                    // across requests.
'keys' => $elements['#cache']['keys'],
                    // The union of the current element's and stored cache contexts.
'contexts' => $redirect_cacheability_updated->getCacheContexts(),
                    // The union of the current element's and stored cache tags.
'tags' => $redirect_cacheability_updated->getCacheTags(),
                    // The union of the current element's and stored cache max-ages.
'max-age' => $redirect_cacheability_updated->getCacheMaxAge(),
                    // The same cache bin as the one for the actual render cache items.
'bin' => $bin,
                ],
            ];
            $cache->set($pre_bubbling_cid, $redirect_data, $this->maxAgeToExpire($redirect_cacheability_updated->getCacheMaxAge()), Cache::mergeTags($redirect_data['#cache']['tags'], [
                'rendered',
            ]));
        }
        // Current cache contexts incomplete: this request only uses a subset of
        // the cache contexts stored in the redirecting cache item. Vary by these
        // additional (conditional) cache contexts as well, otherwise the
        // redirecting cache item would be pointing to a cache item that can never
        // exist.
        if (array_diff($redirect_cacheability_updated->getCacheContexts(), $data['#cache']['contexts'])) {
            // Recalculate the cache ID.
            $recalculated_cid_pseudo_element = [
                '#cache' => [
                    'keys' => $elements['#cache']['keys'],
                    'contexts' => $redirect_cacheability_updated->getCacheContexts(),
                ],
            ];
            $cid = $this->createCacheID($recalculated_cid_pseudo_element);
            // Ensure the about-to-be-cached data uses the merged cache contexts.
            $data['#cache']['contexts'] = $redirect_cacheability_updated->getCacheContexts();
        }
    }
    $cache->set($cid, $data, $this->maxAgeToExpire($elements['#cache']['max-age']), Cache::mergeTags($data['#cache']['tags'], [
        'rendered',
    ]));
}